This invention relates generally to fluid property monitors and monitoring methods. The present invention provides a fluid property monitor and method that use multiple frequency inputs in one or more vibrational modes to produce multiple resonant frequency outputs from which one or more fluid characteristics are determined. A particular application is to determine density of a cement slurry for use in an oil or gas well, but other applications are also contemplated,
Cement slurry is mixed on rigs prior to delivery into wells. A cement slurry typically needs to have a particular density or density range to control downhole pressures during cementing. Slurry density is now typically monitored with nuclear densitometers; however, nuclear sources are forbidden in some countries. Also, density readings are sometimes erratic for foam cements at the wellhead after adding nitrogen. Other densitometers with fewer regulatory requirements (e.g., Coriolis devices) may disrupt flow and suffer from erosion by fluids. Still others (e.g., high frequency acoustic devices) may yield only small depths of investigation into the cross section of a fluid, which is unacceptable for fluids having radial gradients in pipes, for example. In other applications, density, compressibility and viscosity measurements are needed to monitor various flowing fluids, such as stimulation fluids and produced fluids downhole in wells.
A fluid monitor and monitoring method overcoming the aforementioned shortcomings are needed.
The present invention overcomes the above-noted and other shortcomings of the prior art by providing a novel and improved vibrational forced mode fluid property monitor and method. The monitor of the invention preferably imparts vibrations to a distinct system of a conduit and fluid, from which the monitor detects responses resulting from induced deformations of the conduit and the effects on the fluid in the conduit.
The vibrations used in the present invention relate to the modes of vibration of cylindrical shells. Although standing wave patterns for various such modes may be well known, the use and implementation of multiple modes and/or multiple frequency response in determining fluid characteristics as called for by the present invention are new. For a flexural mode, fluids in the bore of shells (to a first approximation) merely add mass loading, resulting in a decrease of resonant frequency. Radial mode resonance is affected by speed of sound which depends on density and compressibility. Torsional mode response couples primarily through the shear viscosity. The Q, the ratio of the center frequency divided by the difference between the upper and lower frequencies having amplitudes 3 dB smaller than the peak amplitude, of any of the resonances relates to energy dissipation, which relates to fluid shear viscosity, fluid bulk viscosity, other fluid dissipation mechanisms and structural energy loss mechanisms. Q""s of the multimodes can be solved by multiple regression to estimate the contributions of each energy loss mechanism.
Such a fluid monitor and method may be used to measure various parameters, including density, compressibility and viscosity (and preferably measured from the same sample volume). Accuracy and response times of the invention preferably equal or exceed those of prior devices for monitoring corresponding fluids and parameters. The presence of multiple resonances permits the use of several center frequencies to improve the statistical accuracy of the determined characteristic. The invention has no nuclear sources, and it avoids device erosion.
The present invention applies to mixing and delivery of cement slurry, for example. As another example, the invention may also apply to evaluating stimulation fluids during delivery for enhanced reservoir production. Additionally, the invention may be used for combining with fluid velocity measurements to develop mass flow meters for various applications, including permanent downhole sensors.
Particular embodiments are described in following sections, but some other advantages of these include the following.
Fixed positions between transducers providing vibrational energy give relatively stable resonant frequencies even if the locations of end clamps supporting the test region conduit change slightly. The use of force couples avoids lateral forces on the end clamps that occur with unbalanced translational forces (forces on end clamps may cause errors in estimating true resonant frequencies, which frequencies are used in determining the desired fluid characteristic(s)).
Separate transducers for transmitting and receiving provide greater noise rejection than combined transmitter/receiver approaches; however, combined transducer embodiments are encompassed in broader aspects of the present invention.
Using a relatively large number of transmitters permits large amplitude vibrations to be generated. The dependence of amplitude on frequency gives the nonlinear behavior of fluid modulii which relates to the amount of gas in liquid. The invention, therefore, may be useful for estimating the amount of gas in liquids.
One embodiment permits transducers to be retrofitted on pipes that are already installed. The invention, therefore, can be designed with no interruption of the flow contours. Interruption of flow contours can cause erosion or improper mixing of multi-phase fluids.
In measuring at least density and using at least the flexural mode, the invention monitors the entire liquid cross section and therefore gives the average density of the total flow.
Another advantage is non-intrusive monitoring of density or other parameters. The radial flow profile in the test region conduit is unaffected by the device. This device requires no curvature of the conduit axis as in curved Coriolis flow meters. The maximum conduit diameter is only limited by the length of the monitor apparatus and the flexural wavelength. The larger the diameter, the longer the monitor apparatus. Active length of the activated conduit can be reduced relative to flexural mode length by using only one or more of the radial modes in implementing the present invention.
One definition of the present invention is as a fluid property monitor comprising a transducer assembly to impart multiple frequency energy to a conduit in multiple modes (e.g., flexural modes and/or radial modes), and to receive resonant frequency energy from the conduit, wherein the resonant frequency energy is responsive to the imparted energy, the conduit and a fluid in the conduit. In a particular embodiment, a radial mode includes a radial hoop mode and a radial oval mode.
The fluid property monitor can also be defined as comprising: a frequency signal generator connected to cause multiple frequency energy to be transferred to a conduit having a fluid to be monitored; and a spectral analysis signal processor connected to receive and process electrical signals generated in response to vibrations propagated through the conduit and the fluid in the conduit in response to transferred multiple frequency energy.
In a particular implementation the present invention provides a densitometer that comprises: at least four transducers disposed circumferentially relative to a location along a length of a conduit connected in-line in a flow path for a fluid to be monitored; a multiple frequency signal generator to provide one or more drive signals for the transducers to impart multiple frequency energy to the conduit; and a controller to connect the one or more drive signals to the transducers such that the transducers are operated to drive the conduit in any of a plurality of modes including a multiple frequency flexural mode and a multiple frequency radial mode.
In another implementation the present invention provides a coherent flow detector comprising first and second fluid property monitors disposed at respective first and second locations along a conduit for a fluid to be monitored. The first fluid property monitor includes a first transducer assembly to impart first multiple frequency energy to the conduit at the first location in multiple modes, including at least a flexural mode and a radial mode, and to receive first resonant frequency energy from the conduit, wherein the first resonant frequency energy is responsive to the imparted first multiple frequency energy, the conduit and a fluid in the conduit. The second fluid property monitor includes a second transducer assembly to impart second multiple frequency energy to the conduit at the second location in multiple modes, including at least a flexural mode and a radial mode, and to receive second resonant frequency energy from the conduit, wherein the second resonant frequency energy is responsive to the imparted second multiple frequency energy, the conduit and the fluid in the conduit.
A method of monitoring a fluid in accordance with the present invention comprises: imparting multiple frequency vibration-inducing energy to a conduit and fluid system, and sensing a plurality of frequency signals from the conduit and fluid system responsive to at least part of the imparted multiple frequency vibration-inducing energy. This method can also comprise: determining at least one characteristic of the conduit and fluid system in response to the sensed plurality of frequency signals. In a particular implementation, determining at least one characteristic includes using an averaging calculation with frequencies identified from the sensed plurality of frequency signals.
A method of monitoring a fluid in accordance with the present invention can also be defined as comprising: imparting multiple frequency energy at a first location of a conduit for a fluid to be monitored; imparting multiple frequency energy at a second location of the conduit; and detecting vibrational motion at a third location of the conduit, wherein the third location is between the first and second locations.
Another definition of the method of monitoring a fluid comprises: flowing a fluid through a conduit; driving at least one of one or more transducers disposed adjacent the conduit to impart energy to the conduit to deform the conduit in a flexural mode; driving at least one of the one or more transducers to impart energy to the conduit to deform the conduit in a radial hoop mode, driving at least one of the one or more transducers to impart energy to the conduit to deform the conduit in a radial oval mode; and generating signals, with at least one of the one or more transducers, in response to the driving steps.
Therefore, from the foregoing, it is a general object of the present invention to provide a novel and improved vibrational forced mode fluid property monitor and method. Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art when the following description of the preferred embodiments is read in conjunction with the accompanying drawings.